This invention relates generally to power generating systems, and, more particularly, to a system and method for controlling load dump voltage of a synchronous machine.
A load dump condition is where the electrical load on the alternator, including the battery, goes instantly to zero, for example, via a broken or disconnected wire. A load dump test determines whether transients, for example, voltage transients output by the alternator under various conditions exceed a predetermined threshold. The threshold is selected to protect, for example, semiconductor devices that may draw power from the system as well as to protect the generator equipment as well. A load dump can, however, result in undesirable transients, for example, a voltage spike in the automobile electrical system which transients can cause damage to sensitive electrical and electronic systems.
Typically a load dump apparatus may operate alone or in combination to short circuit the machine; disconnect the load from the machine; or clamp the voltage to some maximum level. For example, short circuiting the machine, while disconnecting the load including the battery, is an effective method to achieve the load dump in an automobile generator operating at or about the maximum output of the machine. However, if the battery is disconnected the system voltage may collapse and the automobile may stall before the system can reinitiate regulation.
In an exemplary conventional system employing an alternator having a polyphase winding and a bridge circuit, when the load dump condition occurs, avalanche diodes employed in the bridge circuit short the phase windings, thereby limiting output voltage to the avalanche voltage limit of the diode. This limit is within the load dump threshold. However, high energy absorbing diodes are required, which increases the cost of the system.
Hoffman et al., U.S. Pat. No. 6,181,111, disclose an alternator for an automotive vehicle having a rotating permanent magnet (i.e., rotor) configured to induce a voltage in a stationary armature in which a three-phase winding is disposed. The winding is configured in a delta arrangement and is coupled to a bridge comprising six controlled rectifiers, an upper and lower rectifier for each phase. The rectifiers are disclosed as being silicon controlled rectifiers (SCRs). The alternator may be configured to produce a conventional output voltage of about 14.5 volts. Hoffman et al. further disclose a relatively complex circuit for responding to a load dump condition. The complex circuit increases cost and limits its usefulness in large scale production. Moreover, Hoffman et al. recognize that a high transient voltage may nonetheless exist for one alternation, notwithstanding the presence of the above complex circuit, and therefore provide for a metal oxide varistor (MOV) across the alternator output terminals as a limiting device. It is believed that such a device would have to be selected to dissipate a large amount of power and accordingly would be of increased cost which is undesirable.
Peter, U.S. Pat. No. 5,773,964, discloses an apparatus for controlling the output and torque of a synchronous alternator and employs a control bridge rectifier and a pulse width modulator (PWM) signal controlled field winding. The bridge in Peter is controlled for establishing a desired phase relationship between the alternating voltage and alternating current of the output winding to control the output and torque of the machine. While Peter discusses no load and short circuit conditions, Peter is concerned primarily with the control of the inductive characteristics of the circuit in order to vary the output and control the torque load on a machine.
There is therefore a need for an improved power generating system that minimizes or eliminates one or more of the problems discussed hereinabove.
The invention is based upon the discovery that, in an automotive electrical system employing a synchronous generator having polyphase windings, a field winding and a bridge having upper and lower electronically controlled switches operably connected to the polyphase windings, when an over-voltage is detected in any phase, the field is set to a predetermined level, the switches in the upper rectifier are set open so that the upper portion of the bridge operates as a diode bridge, and the lower switches are controlled in accordance with the corresponding current and voltage for the phase to which it is connected, so that each lower switch is off or open for a positive going part of the phase current, and when the phase current voltage exceeds a selected triggering level, the switch for the corresponding phase turns on shorting the phase winding for the remaining portion of the cycle. When the voltage falls below the trigger level the lower switch is turned off.
In an exemplary embodiment the invention is directed to a power generating system comprising a synchronous machine including a multiphase stator winding output and corresponding outputs, and a field winding; a bridge including a pair of electronically controllable upper and lower switches for each phase coupled to the corresponding stator winding output and having a common node, the bridge having an output configured to be coupled to the battery and load of an automobile. A switch controller is operatively connected to open and close the switches to control the bridge output for operation at a predetermined first voltage corresponding to an operating voltage, and in response to an over-voltage condition to close the lower switches to control the bridge at a voltage lower than a first voltage. Concurrently with the over-voltage condition, the duty cycle of the field winding may be set to a predetermined level to insure that the system voltage is maintained at some minimum level. The invention is also operable for use in a permanent magnet synchronous machine.
The invention may also be characterized as a load dump for a power generating system employing a synchronous machine having polyphase output windings producing a polyphase output and which includes a controlled rectifier bridge having an input coupled to the polyphase output windings and an output coupled between a direct voltage output terminal and ground terminal of the bridge. The bridge employs a plurality of electronically controllable switches, each being controllable to respective on and off conditions an accordance with a corresponding conduction signal applied thereto. The plurality of switches are arranged as series connected pairs between the output terminal and ground and have an intermediate node for each pair coupled to the polyphase winding. A switch controller responsively coupled to the output terminal of the bridge produces selected over-voltage conduction signals operative to turn off the upper switches and to turn on the lower switches when the output voltage is greater than or equal to a selected over-voltage condition, and to extinguish the over-voltage conduction signals when the output voltage falls to a level which is less than or equal to a nominal operating voltage below the over-voltage condition.